The Wolfram Functions Site functions.wolfram.com contains the largest collection of identities for elementary and special functions ever assembled. The site is generated from a set of Mathematica notebooks with typeset versions of all identities. The notebooks contain about 90,000 mathematical formulas. Because Mathematica notebooks are structured ASCII files that can be processed and manipulated programmatically by the Mathematica kernel, Mathematica can read and "understand" the formulas. Therefore, Mathematica can completely analyze and classify all the identities with respect to their mathematical structure and the functions that occur in them. The results of this analysis allow us to build a semantic search engine for mathematical identities. I will discuss the backend of the current mathematical search interface deployed on the Wolfram Functions site.